Nuclear radiation detector with control grid



Jan. 18, 1966 H. P. SPRACKLEN 3,230,372

NUCLEAR RADIATION DETECTOR WITH CONTROL GRID Filed Aug. 9, 1963 2 Sheets-Sheet 1 INVENTOR. HOWARD P SPRACKL EN ATTORNEY Jan. 18, 1966 Filed Aug. 9, 1963 H. P. SPRACKLEN 3,230,372

NUCLEAR RADIATION DETECTOR WITH CONTROL GRID 2 Sheets-Sheet 2 INVENTOR. HOWARD P SPRACKLEN AT TORNE Y United States Patent 3,230,372 NUCLEAR RADIATION DETECTOR WITH CONTROL GRID Howard P. Spracklen, Castro Valley, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 9, 1963, Ser. No. 301,214 6 Claims. (Cl. 250-83.6)

The present invention relates to ionization chambers, and, more particularly, to an improved ionization chamber having at least three elements, capable of detecting and measuring a wide range of radiation intensity levels.

In general, conventional ionization chambers consist of two electrodes separated by a volume of gas. Radiation flux incident on an ionization chamber will ionize the gas molecules. The negatively charged ions produced will travel parallel to the electric field to the electrode at the higher electropositive potential, called the anode, while the positively charged ions will travel in the opposite direction to the electrode at the lower potential called the cathode. Both of these electrodes are referred to as collectors. The resulting current, or reduction of the collector potentials, is a measure of the intensity of the ionizing radiation flux incident on the chamber volume.

An ion traveling toward an electrode is apt to collide with other particles. Such an ion may recombine with an ion of opposite charge to form a neutral particle or if sufficiently energetic, may further ionize other gas molecules with which they collide. In the latter case, the ion current arriving at the collectors will increase while in the former case the ion current decreases. Hence, the ion current intensity depends on the existing electric field strength in the chamber, since the ion velocity and electric field strength are in direct relationship to each other.

Thus, a high potential difference between anode and cathode will render an ionization chamber more sensitive, enabling detection of lower radiation intensity levels, whereas a smaller potential difference shifts the measurable range toward higher radiation intensities. The lower detection limit for a given potential difference depends on the sensitivity of the circuitry to detect small ion currents in the chamber. The upper limit of detection at a given potential difference between anode and cathode is reached, when an increase in the radiation intensity no longer produces an increase in the ion current flowing through the tube. While this increase in radiation intensity is still associated with an increase in the number of ion pairs produced in the chamber, they will not reach the electrodes since the capacity to collect ions has been reached V at one of the electrodes. This effect is commonly called saturation. Hence, of the simple, conventional ionization chambers which use a constant potential difference between anode and cathode, those having a high potential difference between anode and cathode have measuring ranges effective at lower radiation intensity levels, and conversely, a smaller potential difference between the anode and cathode allows measurement of higher radiation intensities but at the expense of sensitivity to low intensity radiation.

In view of the importance of radiation diagnostics due to the expanse of nuclear reactor construction, problems of radioactive waste disposal and, not least, the needs of a civil defense system capable of dealing with the radiation problems of nuclear warfare, an ionization chamber with a greater measuring range is highly desirable.

Various concepts have been utilized in developing instruments operating on the ionization chamber principle and having a measuring and detection range large enough to be useful for the above-stated purposes.

One such concept involves the placing of a number of ionization chambers in series, thus enabling the effective volume of the chamber to be enlarged by switching from one singular chamber to a multiplicity of chambers for high sensitivity measurements.

Another method of measuring radiation over a large range is to employ a device having two gas spaces, one large compared to the other. This device features three electrodes which enable one to determine the radioactivity level by measuring the potentials of the three electrodes after exposure to radiation and then comparing the potential reduction data with a set of predetermined calibration curves.

Still another instrument uses a trumpet shaped cathode. A wire collector is situated along the axis of the volume defined by the cathode. The increasing distance between the two electrodes gives rise to an electric field gradient along the tube. Alteration of the potential difference between the electrodes changes the sensitive volume of the instrument and provides the necessary extension of the measuring capacity of the device.

It is evident from the description of the above ionization chambers that they possess certain disadvantages. In particular, the first mentioned device is bulky and not amenable to automatic readings. The second device is equally unsuited to give instantaneous readings; in fact, care must be taken to prevent multiple and prolonged exposures since the potential reduction of the electrode voltages is cumulative.

The last device is not a constant potential ionization chamber. It has a cathode which is the locus of the surface of revolution defined by a mathematical expression, an unusual shape which is not easily realized.

Compared to these devices, the present invention is not only completely different, but also superior in function and simplicity of design. The invention provides an ionization chamber, which features an auxiliary collector electrode disposed in the effective volume of the gas chamber. This auxiliary collector electrode extends the measuring range by intercepting a variable fraction of the ion current traveling to the collector electrode of lowest capacity thus preventing saturation conditions from occurring there.

Accordingly, it is an object of the present invention to provide an ionization chamber with a greatly extended sensitivity range.

Another object of the present invention is to give instant readings of a radiation intensity.

A further object of the invention is to provide an ionization chamber which is easily portable.

Still another object of the present invention is to provide an ionization chamber which is relatively simple and economical to construct.

Other objects and advantages will be apparent to those skilled in the art upon a consideration of the following description and the attached drawing, wherein:

FIGURE 1 is a cross-sectional view of a preferred embodiment of the invention together with a diagrammatical view of an external circuit arrangement;

FIGURE 2 is a perspective view of a helical auxiliary collector electrode;

FIGURES 3 and 4 are perspective views of alternative electrode arrangements; and

FIGURE 5 is a cross-sectional view of an ionization chamber of the invention together with a diagrammatical view of an alternative external circuit arrangement.

Referring now to FIGURE 1, the numeral 11 designates a cylindrical enclosure having a substantially hemispherical end section 12 and a flat base 13. Enclosure 11 is substantially transparent to the radiation to be detected and is hermetically sealed to confine a gas 14 which ionizes upon exposure to radiation. Disposed in spaced relationship within the enclosure are a cathode 16, an anode 17, and an auxiliary collector electrode 18. The cathode lines the entire surface of the enclosure 11 except for the base 13. The cathode 16 is connected to the exterior circuitry through the base by a lead 19. Anode 17 is disposed axially in the enclosure 11, its free end being located at a point about equidistant from all points of the hemispherical end section 12 of the enclosure 11. The auxiliary collector electrode 18 surrounds anode 17 and consists of a conducting grid with a sieve-like structure, such as a fine mesh conductive screen. Anode 17 and auxiliary collector electrode 18 are connected to outside circuit components by leads 21 and 22 similarly as the cathode. Lead ducts 23 through the base are closed with suitable non-conducting seals 24 to maintain enclosure 11 vacuum tight.

The negative terminal 26 of a power supply 27 is connected to the cathode 16 of the ionization chamber. The positive terminal 28 is connected to ground as represented at 29. The anode 17 is connected to the input of a high gain amplifier 32. A resistor 33 is electrically connected between the input of the amplifier 32 and ground 29. The output of amplifier 32 is electrically connected to the auxiliary collector electrode 18 of the ionization chamber. A meter 36, responsive to the output voltage of the amplifier 32 is electrically connected between the output of the amplifier and ground 29.

To illustrate the function of the device, imagine radiation flux represented at 37 as passing into the ionization chamber and ionizing the gas contained therein. Power supply 27 establishes a constant potential difference between cathode 16 and anode 17. The negatively charged gas ions produced by the radiation travel to the anode 17 and the positive ions travel to the cathode 16. This motion of charged particles gives rise to a current, which in flowing through the resistor 33 develops a potential difference across it rendering the input to the amplifier more negative. Amplifier 32 responds to this charge by providing an amplified voltage signal to the auxiliary collector electrode 18. The output signal of the amplifier is of opposite polarity than the input signal, i.e., the voltage across resistor 33, and alters the potential of the auxiliary collector electrode. The overall result of this degenerative feedback system is to render the auxiliary collector electrode more positive as the ion current increases. But the potential drop across the resistor 33 is proportional to the ion current, and hence to the radiation exposure of the ionization chamber. It follows that with increasing exposure to radiation the share of the negative ion current collected by the auxiliary collector electrode 18 increases. This action greatly increases the radiation intensity threshold of saturation of the anode. The voltmeter 36 registers the output of the amplifier and may be calibrated in radiation units to register the radiation intensity level instantaneously.

It is to be understood that the auxiliary collector electrode structure is not limited to the grid-like structure shown in FIG. 1. In general, any conductor capable of responding to a voltage change and capable of intercepting a fraction of the ion current traveling toward the low capacity electrode, which may be the anode or the cathode and will be referred to as collector.

Alternate preferred control electrode arrangements are illustrated in FIGURES 2, 3, and 4.

In FIG. 2, the auxiliary collector electrode 41 is a helical conductor surrounding a central collector.

FIG. 3 shows an arrangement wherein a collector 52 is not concentric to the rod-like auxiliary collector electrode 53.

FIG. 4 shows an arrangement of multiple rod-like auxiliary collector electrodes 63 disposed about a central collector 62.

It is also to be understood that the important feature of the present ionization chamber resides in an auxiliary collector electrode, Whose potential difference with respect to the collector electrode of lowest collection capacity is variable. While in the chamber of the above description the potential difference between the cathode and anode is fixed and the potential difference between auxiliary collector electrode and anode is variable, the invention includes an ionization chamber having an auxiliary collector electrode, the potential difference of which is constant with respect to the electrode of larger surface area, and a smaller electrode of variable potential with respect to the auxiliary collector electrode. Such an embodiment is illustrated in FIGURE 5. Referring now to FIG. 5, an electrometer tube 71 has its anode 72 connected to the anode 73 of the ionization chamber and the positive terminal of a battery 74, constituting the power supply of the circuit. The cathode 76 of the ionization chamber is connected to the grid 77 of the electrometer tube 71. A biasing resistor 79 and a biasing battery 81 are connected in series between the grid 77 and the cathode 78 of the electrometer tube with the positive terminal of the biasing battery being directly connected to the cathode 78. The auxiliary collector electrode 82 of the ionization chamber is connected to the negative terminal of battery 74 and also to the positive terminal of a second biasing battery 83. The negative terminal of the battery 83 is electrically connected to the cathode 78 of the electrometer tube and the positive terminal of a battery 81 through a load resistor 84 in a series with a meter arrangement, formed by a battery 87, a variable resistor 88, and an ammeter 86. The positive terminal of the battery 87 is connected directly to the resistor 84. Connected in parallel with battery 87 and the variable resistor 88 is an ammeter 86. The anode 73 of the ionization chamber forms a cylindrical envelope, which contains a gas. The cathode 76 is a cylindrical conductor which is disposed in the center of the cylindrical envelope of the cathode 73 and is surrounded by a helical auxiliary collector electrode such as shown in FIG. 2.

The circuit of the above embodiment is of the cathode follower type. As the radiation intensity increases, more ion current will flow in the ionization chamber. The change in current flow through the resistor 79 biases the grid of the electrometer tube more positive with respect to the cathode of the tube. As a result of this change in bias more current will flow through the tube thus causing the potential of the cathode to become more positive. The collection of positive ion current at the cathode is thereby decreased. By this action the approach to saturation conditions at the cathode is effectively retarded. The current flow through the electrometer tube is monitored by the ammeter which is calibrated in radiation units.

EXAMPLE An ionization chamber in accordance with the embodi ment shown in FIG. 5 was constructed utilizing the specific circuit components given in Table I and an ionization chamber of dimensions given in Table II.

Table I CIRCUIT COMPONENTS Ammeter (range) microamps 0-20 Table II IONIZA'IION CHAMBER Gas Air Diameter of anode in 4 length of anode in 5 Diameter of cathode in /2 Length of cathode in 4 Number of turns of the control electrode helix The current readout of the instrument is calibrated and checked by exposure to ionizing radiation sources of known intensities. To measure the ionizing radiation intensity for example 7 radiation intensity in any one place it is merely necessary to physically place the easily portable device of the present embodiment into that area. Radiation incident on the chamber will render the chamber conducting and the circuitry will adjust the current flowing through the ammeter automatically. The ammeter will give readings of this current and hence the radiation intensity instantaneously. An X-Y recorder may be employed in place of the ammeter, and both instantaneous intensities as well as integrated overall doses may be evaluated therefrom. Upon so first calibrating the instrument against known intensities of such radiation, the indicated current serves as a measure of radiation intensity. The order of the radiation intensities measurable with the device of this embodiment extends from the millirem to the kilorem range.

Although the above description deals with specific embodiments, it should be understood that the invention may be realized in many modifications, especially with regard to the external circuitry. Thus, for example, the voltage impressed on the auxiliary collector electrode may be controlled by a switching mechanism which increases the voltage impressed on the grid by discrete amounts. Switching may be triggered by the output of the anode when it exceeds a value near the saturation point. The increase of the auxiliary collector electrode potential may be of such a value as to depress the fraction of the current incident on the anode to a value near the lower detection limit of the instrument. The now multivalued function of ion current arriving at the anode vs. the radiation intensity is easily separated into decades according to the distinct grid potential values.

It is especially to be understood that the auxiliary collector electrode may be located outside the region which is spacially intermediate the anode and cathode and must only be disposed within the sensitive volume of the ionization chamber to function effectively.

Just as the occurrence of the condition of saturation sets an upper limit to the radiation intensity which can be measured with two element ionization chambers, so does the present instrument seize to indicate radiation intensities which produce ion populations exceeding the collection capacity of the electrode of lowest capacity and the auxiliary collector electrode combined.

Hence, as a last example of an obvious modification, the disclosed device could be outfitted with other additional auxiliary collector electrodes which could further aid in the collection of the ion current by imposition of a potential dependent on the ion current arriving at the anode and primary control electrodes. Thereby the upper limit of measurable radiation intensity could be even further extended for especially high intensity radiation measurements.

In view of the above and numerous other equally possible arrangements, the scope of the invention should be considered limited only by the following claims.

What is claimed is:

1. In an ionization chamber containing a gas ionizable by radiation and having cathodic and anodic collector electrodes, the combination therewith of at least one auxiliary ion current collector electrode disposed in spaced relationship to said anodic and cathodic collector electrodes in the sensitive volume of said ionization chamber, and a variable voltage source means connected to at least one of said cathodic and anodic collector electrodes and responsive to the ion current arriving thereat to establish a potential difference responsive to the ion current level between the auxiliary collector electrode and at least one of said cathodic and anodic collector electrodes which is eifective to maintain said ion current incident on said cathodic and anodic collector electrodes below the saturation value of said collector electrodes.

2. The combination of claim 1 wherein the auxiliary collector electrode is a sieve-like conductor.

3. The combination of claim 1, wherein the auxiliary collector electrode is a helical wire which surrounds a central collector electrode.

4. The combination of claim 1, wherein the auxiliary collector electrode comprises at least one rod-like conductor.

5. In an ionization chamber containing a gas ionizable by radiation and having cathodic and anodic collector electrodes, the combination therewith of at least one auxiliary ion current collector electrode disposed in spaced relationship to said anodic and cathodic collector electrodes in the sensitive volume of said ionization chamber, and means for adjusting the potential difference of said auxiliary collector electrode with respect to at least one of said cathodic and anodic collector electrodes, said means being directly responsive to the ion current arriving at one of said collector electrodes, and comprising a high gain amplifier, the input terminal of which is connected to said anodic collector electrode and the output terminal connected to said auxiliary collector electrode, a resistor connected between said anodic collector electrode and ground, a voltage measuring means electrically connected between ground and the output terminal of said amplifier, and a voltage source electrically connected between said anodic and cathodic collector electrodes.

6. In an ionization chamber containing a gas ionizable by radiation and having cathodic and anodic collector electrodes, the combination therewith of at least one auxiliary ion current collector electrode disposed in spaced relationship to said anodic and cathodic collector electrodes in the sensitive volume of said ionization chamber, and means for adjusting the potential difference of said auxiliary collector electrode with respect to at least one of said cathodic and anodic collector electrodes, said means being directly responsive to the ion current arriving at one of said collector electrodes, and comprising a cathode follower circuit employing at least a three element vacuum tube, the control grid of said tube being electrically connected to one of the cathodic and anodic collector electrodes, a DC. power supply, the negative terminal of which is electrically connected to said auxiliary collector electrode and the cathode circuit of said cathode follower circuit, and the positive terminal being connected to the anode of said tube and the other of said cathodic and anodic collector electrodes of said ionization chamber.

References Cited by the Examiner UNITED STATES PATENTS 2,470,920 5/1949 Colson 313-93 X 2,480,808 8/1949 Fearon 25083.6 2,759,118 8/1956 Ruble 31393 3,047,761 7/1962 Howling 31393 3,067,350 12/1962 Stebler et al. 25083.6 X

RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner.

M. ABRAMSON, Assistant Examiner. 

1. IN AN IONIZATION CHAMBER CONTAINING A GAS IONIZABLE BY RADIATION AND HAVING CATHODIC AND ANODIC COLLECTOR ELECTRODES, THE COMBINATION THEREWITH OF AT LEAST ONE AUXILIARY ION CURRENT COLLECTOR ELECTRODE DISPOSED IN SPACED RELATIONSHIP TO SAID ANODIC AND CATHODIC COLLECTOR ELECTRODES IN THE SENSITIVE VOLUME OF SAID IONIZATION CHAMBER, AND A VARIABLE VOLTAGE SOURCE MEANS CONNECTED TO AT LEAST ONE OF SAID CATHODIC AND ANODIC COLLECTOR ELECTRODES AND RESPONSIVE TO THE ION CURRENT ARRIVING THEREAT TO ES- 